1. Field of the Invention
The present invention relates to a plastic lens and a process for the production thereof.
2. Description of the Related Art
Recently, a plastic lens has been widely used as a lens for spectacles or cameras because of its lightness, excellent impact resistance and dyeability as compared with an inorganic glass lens.
In general, a plastic lens has been conventionally produced in accordance with a casting method in which a monomer liquid is poured into a glass or metallic mold that has previously been coated with a release agent, polymerized and hardened and then the lens is removed from the mold. After washing, the lens is first provided with a hard coat film, and then with an anti-reflecting film.
However, coating of each mold with a release agent is very cumbersome and reduces working efficiency. On the other hand, use of an internal release agent causes a milky turbidity.
With respect to methods of forming an anti-reflecting film and a hard coat film, there have been known two methods; namely, a method of coating and hardening organic substances and a vacuum deposition method utilizing inorganic substances.
In the method of coating and hardening organic substances, the plastic lens is coated with a hard coat solution. However, it is difficult in this method to obtain a hard coat film which adheres strongly to the surface of the lens, because wettability may vary depending upon the construction materials of the lens, as well as the hard coat solution. It is already known that the surface of the lens may be pre-treated for example, with an activated gas or a chemical agent, in order to improve the adhesion between the plastic lens and the hard coat film. However, such treatment is troublesome. Moreover, this method has a further disadvantage in that it is necessary to carry out the heat treatment of the hard coat solution under a restricted temperature range, since most plastic lenses are yellowed or distorted at a temperature above 120.degree. C.
On the other hand, a vacuum deposition method can give a dense hard film. However, since this film lacks flexibility, it may be cracked by temperature changes, when there is a difference in expansion coefficient between the base material of the lens and the film. Further, the plastic lens cannot be heated to a sufficiently high temperature to achieve vacuum deposition because of water absorption properties of plastics, so that an anti-reflecting film having a constant refractive index and a definite reflection color cannot be obtained. In addition, this method has additional drawbacks in that it is both expensive and lacking in productivity, because exhaust must be continued for at least one hour to achieve a degree of vacuum of 10.sup.-5 torr.
As described above, since most plastic lenses are yellowed or distorted at a temperature above 120.degree. C., substances requiring a heat treatment at a high temperature for the deposition cannot be used. For example, MgF.sub.2 which is conventionally used as a component for a film of a low refractive index cannot be employed as an anti-reflecting film component for a plastic lens, because MgF.sub.2 must be heated to about 200.degree. C. or more.